1. Field of the Invention
The present invention relates to a mobile communication terminal, and more particularly to a method and apparatus for reducing power consumption of a mobile communication terminal in a spread spectrum communication system such as a CDMA (Code Division Multiple Access) cellular mobile communication system.
2. Description of the Related Art
It is well known in the art that a spread spectrum communication system such as a CDMA cellular mobile communication system divides the whole service area into a plurality of cells, manages the cells by many base stations (BSs) using the same radio frequency (RF), and centrally manages the base stations by a MSC (Mobile Switching Center). The base stations are respectively assigned unique PN (Pseudo-random Noise) spreading codes so that the MSC can readily identify each base station.
In such a CDMA cellular mobile communication system, a base station transmits signals to the outside via a pilot channel, a synchronous channel, and a paging channel as well as a traffic channel for supporting the exchange of voice or data. The pilot channel transmits signals of a predetermined level, which are composed of the same bit values and are covered by a Walsh code “0”, and is used for CDMA cellular mobile communication system identification, system acquisition initialization, standby state hand-off, and coherent demodulation of demodulation/paging/traffic channels. Particularly, the mobile communication terminal acquires a pilot symbol from the pilot channel, and determines on the basis of the acquired paging symbol which one of many paging channels received from neighbor base stations is to be demodulated.
FIG. 1 is a flow chart illustrating a conventional procedure of monitoring a paging channel.
As shown in FIG. 1, a mobile communication terminal provides a communication circuitry needed to manage a radio signal with a power-supply voltage after receiving a wake-up signal at step S110. As known in the art, the communication circuitry contains a radio frequency (RF) unit, a demodulator having a searcher, and a DSP (Digital Signal Processor) having a call control processor. After the mobile communication terminal has been woken up at step S110, the mobile communication terminal acquires a pilot symbol on a pilot channel from a base station by means of the communication circuitry at step S120. After the pilot symbol on the pilot channel has been acquired at step S120, the mobile communication terminal establishes a synchronization acquisition to achieve time-alignment with the base station at step S130.
After the synchronization acquisition has been made at step S130, the mobile communication terminal monitors all paging channels successively (i.e., non-slotted mode) or periodically (i.e., slotted mode) at step S140, and determines at step S150 whether a paging channel message is detected or not on the basis of the monitoring result. In the case where the paging channel message has been detected at step S150, the mobile communication terminal processes the detected paging channel message at step S180. But, in the case where no paging channel message has been detected at step S150, the mobile communication terminal determines at step S160 whether all valid paging channels are completely monitored. If it is determined at step S160 that all valid paging channels have not been completely monitored, the mobile communication terminal tunes a current paging channel to another paging channel at step S170 and returns to step S140. In the case where it is determined that no paging channel message has been detected at step S150 and all valid paging channels have been completely monitored at step S160, or the paging channel message has been completely processed at step S180, the mobile communication terminal cuts off the power-supply voltage for the communication circuitry by entering a sleep mode at step S190.
As for a CDMA mobile communication terminal such as the above mobile communication terminal of FIG. 1, the mobile communication terminal is operated at a slotted mode during a predetermined period of time where a call connection state is not made, thereby increasing a standby time and a system efficiency. In the slotted mode, a unique timeslot is assigned to every mobile communication terminal, and then the mobile communication terminal is woken up within its own timeslot to monitor a paging channel of a base station but remains in a sleep mode in the remaining timeslots. The slotted mode may considerably reduce power consumption of the mobile communication terminal in the standby state because the communication circuitry of the mobile communication terminal needs not receive a power-supply voltage in the slotted mode.
FIG. 2 is a view showing conventional paging channel slots of the CDMA mobile communication system.
Referring to FIG. 2, the conventional paging channel is composed of 16 paging channel slots that are periodically repeated. Three such slots, N−1, N and N+1 are shown in FIG. 2. Each paging channel slot has a time period of 80 ms. During the time period of 80 ms, a rollover period of 26.667 ms of a PN (pseudo-noise) generator can be thrice repeated and one frame of 20 ms can be repeated four times. Each paging slot is composed of 64 pilot channel groups (PCGs) that respectively contain 18 pilot symbols.
In case of the above paging channel, each mobile communication terminal is allocated to one predetermined slot among the 16 paging channel slots, and is woken up before its own allocated slot so it may monitor its own allocated corresponding paging channel slot. The mobile communication terminal remains in a sleep mode in the remaining paging channel slots. Such a mobile communication terminal is called “slotted mode mobile communication terminal”.
The slotted mode mobile communication terminal determines a sleep mode entering timing on the basis of the rollover period of 26.667 ms of a PN code of a PN generator in a CDMA mobile communication system. In other words, a control unit of the mobile communication terminal generates periodic PN roll signals in response to the PN code rollover period of the PN generator, resulting in a power-off state of the communication circuitry. After an elapsing of the sleep mode, the mobile communication terminal is woken up on the basis of the periodic PN rollover signals. The reason why the mobile communication terminal enters the sleep mode or the wake-up mode on the basis of the PN code rollover period (i.e., a period of the PN rollover signals) is that the mobile communication terminal may quickly establish synchronization re-acquisition with a base station because the PN generators of the mobile communication terminal consistently enter the sleep mode and wake-up mode based on the timing of the PN code rollover period.
In this case, in order to enter the sleep mode after determining that there are no paging channel message in the allocated paging channel or after processing a paging message, the control unit of the mobile communication terminal prepares hardware setup parameters needed to enter the sleep mode, finishes a hardware setup of the communication circuitry for the sleep mode, and commands the communication circuitry to enter the sleep mode. As a result, the communication circuitry is actually in the sleep mode after all the hardwares have been completely set up. Therefore, even if the control unit of the mobile communication terminal determines the sleep mode and commands the communication circuitry to enter it, the sleep mode initiation timing of the communication circuitry may be delayed by a period of time that can equal the rollover period of 26.667 ms. As a result, the wake-up mode initiation timing of the communication circuitry is prior to a paging slot initiation timing by one PN rollover period.
FIG. 3 is a view showing a conventional sleep mode initiation timing of a mobile communication terminal.
As shown in FIG. 3, a mobile communication terminal monitors a paging channel during its own allocated paging slot. In the case where no message for calling the mobile communication terminal is detected in the monitoring step, the control unit of the mobile communication terminal sets up hardware parameters for the sleep mode on the basis of an initiation timing of the next paging slot to be monitored. In other words, at a predetermined time at which the sleep mode has to begin, the control unit generates a sleep mode command signal S0.
After that, if the mobile communication terminal finishes the setup of all hardware parameters for the sleep mode, it enters the sleep mode on the basis of internal period signals (i.e., PN rollover signals) S1, S2, S3, S4, S5, S6 and S7. In the case where the control unit generates a PN rollover signal before completion of set-up of the hardware parameters after the sleep mode command signal S0, the mobile communication terminal establishes a synchronization with a subsequent PN rollover signal (that is, S5 instead of preceding PN rollover signal S4 next to the sleep mode command signal S0), and then enters the sleep mode. As noted above, a power-supply voltage is not applied to the communication circuitry such as an RF unit, demodulator, and a call control processor in the sleep mode in order to reduce power consumption, that is, the mobile communication terminal enters POWER DOWN state in the sleep mode. Thereafter, the mobile communication terminal is woken up by establishing synchronization with a PN rollover signal S6 that is prior to the next paging slot to be monitored by one PN rollover period. Thus, the mobile communication terminal provides the communication circuitry with a power-supply voltage to enter POWER UP state.
However, the above-mentioned paging channel monitoring operation of the mobile communication terminal has a disadvantage in that the time period in which the mobile communication terminal is in the wake-up mode becomes unnecessarily longer. In particular detail, the timing at which the communication circuitry of the mobile communication terminal begins to enter the sleep mode corresponds to the timing of the PN rollover signal S4 next to the sleep mode command signal S0. But the communication circuitry actually establishes synchronization with the PN rollover signal S5 rather than the PN rollover signal S4 because of undesired time delay caused by many processes required for powering off itself, and then enters the sleep mode. As a result, the mobile communication terminal unnecessarily consumes excessive power during at least part of one PN rollover period of S5 to S4. Also, after that, even when the mobile communication terminal is woken up, it establishes synchronization with the PN rollover signal S6 prior to an initiation timing (i.e., a PN rollover signal S7) of the next paging slot, and is then woken up. Consequently, the mobile communication terminal also unnecessarily consumes power during at least part of one PN rollover period of S7 to S6.
Accordingly, it is desirable that, the sleep mode initiation timing of the mobile communication terminal be faster and the wake-up mode initiation timing be delayed in such a way that the undesired power consumption is minimized, resulting in reduction in the length of the wake-up mode period of the terminal.